Secondary 4 Combined Science Biology Preliminary Examination Paper 2

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Combined Science Biology Secondary 4 — Version 2: Answer Key

Total Marks: 65
Section A: 28 marks
Section B: 22 marks
Section C: 15 marks

Section A Answers

1. [1 mark]
Answer: Diffusion
Teaching note: Oxygen moves from high concentration in the lungs/alveoli to lower concentration in the blood and then the muscle cell by diffusion. It is passive (no energy needed). Common mistake: writing "active transport" or naming a structure instead of the process.

2. [2 marks]
Answer: Muscle cells have more mitochondria (3000 vs 200) because they require more ATP for contraction; skin cells have lower energy demand so fewer mitochondria.
Mark breakdown: 1 mark for stating muscle has higher count and skin lower; 1 mark for linking to energy/ATP need.
Teaching note: Mitochondria are sites of aerobic respiration producing ATP. More active cells need more ATP, thus more mitochondria.

3. [2 marks]
(a) [1] Chloroplast
(b) [1] Photosynthesis / makes food (glucose) using light energy.
Teaching note: From the <image_placeholder> Q3-fig1, X is the green oval = chloroplast. Function is conversion of light energy to chemical energy.

4. [2 marks]
Answer: Red blood cells lose nucleus to create more space for haemoglobin to carry oxygen; liver cells need nucleus to control cell activities (e.g., protein synthesis, metabolism).
Mark breakdown: 1 mark each for correct reason for each cell type.

5. [1 mark]
Answer: Diffusion does not require energy; active transport requires energy (ATP). OR diffusion is down gradient, active transport against gradient.
Teaching note: Key distinction for membrane transport.

6. [1 mark]
Answer: Protein channel (or channel protein).
Teaching note: From Q6-fig1, the spanning protein is the channel allowing selected molecules through the bilayer.

7. [2 marks]
Answer: Incorrect. Correct: Osmosis is the diffusion of water from a region of HIGH water potential to a region of LOW water potential (or from dilute to concentrated solution).
Mark breakdown: 1 mark identifying wrong; 1 mark correct statement.

8. [2 marks]
Answer: During S phase, DNA is replicated so chromosome number stays same but DNA content doubles; skin cell is in G1 (unreplicated).
Mark breakdown: 1 mark for S phase replication; 1 mark comparing to skin cell G1.

9. [1 mark]
Answer: Active transport.
Teaching note: Against concentration gradient requires ATP.

10. [1 mark]
Answer: Carbon dioxide.
Teaching note: At night no photosynthesis; respiration releases CO₂ to atmosphere.

11. [2 marks]
(a) [1] Red blood cell (erythrocyte).
(b) [1] Biconcave shape / no nucleus (either from diagram).
Teaching note: Q11-fig1 shows biconcave non-nucleated red-tinted cell.

12. [2 marks]
Answer: Ribosomes are sites of protein synthesis; many ribosomes mean the cell synthesises large amounts of protein.
Mark breakdown: 1 mark function, 1 mark inference.

13. [1 mark]
Answer: Osmosis.

14. [2 marks]
Answer: Long thin extension increases surface area for absorption of water and minerals from soil.
Mark breakdown: 1 mark increase SA, 1 mark link to absorption.

Section B Answers

15. [4 marks]
(a) [1] Palisade mesophyll.
(b) [3] Palisade has ~50 chloroplasts because it is the main photosynthetic layer receiving most light; spongy has ~20 as it also photosynthesises but with more air spaces for gas exchange, less packed with chloroplasts. Upper epidermis has 0 as it is transparent, not for photosynthesis.
Mark breakdown: 1 mark tissue comparison, 2 marks functional explanation.

16. [4 marks]
(a) [2] Pancreatic cell; very many ribosomes and it secretes proteins (enzymes).
(b) [2] Bacteria are prokaryotes; they do not have membrane-bound organelles like mitochondria; they respire using cell membrane.
Mark breakdown: 1+1 each part.

17. [3 marks]
Answer: Oxygen diffuses from alveolus (high pO₂) across alveolar epithelium and capillary wall into plasma then into red blood cell; binds haemoglobin.
Mark breakdown: 1 mark diffusion, 1 mark crossing barriers, 1 mark into RBC/binding.

18. [2 marks]
Answer: Net water movement is OUT of the cell (from solution -200 to cell -500? Wait: water moves from higher ψ to lower ψ; solution is -200 (higher) to cell -500 (lower) → INTO cell). Correct: into the cell.
Teaching note: Water potential more negative = lower. So -200 > -500, net into cell. Common mistake: reversing direction.

19. [2 marks]
Answer: Cell P is likely secretory/active in transport; more vesicles for storing/transporting materials.
Mark breakdown: 1 mark observation, 1 mark function link.

Section C Answers

20. [15 marks]
Answer (full pathway):

1. Inhaled air enters trachea → bronchi → bronchioles → alveoli (lung). [2]
2. O₂ diffuses across alveolar epithelium and capillary endothelium into blood plasma. [3]
3. O₂ binds haemoglobin in red blood cells in pulmonary capillary. [2]
4. Blood leaves lung via pulmonary vein → left atrium → left ventricle → aorta → femoral artery → leg capillary. [4]
5. O₂ diffuses from capillary into tissue fluid then into muscle cell membrane → mitochondrion. [4]

Mark descriptors:

• Named structures (alveoli, capillary, heart chambers, arteries) = up to 6 marks
• Processes (diffusion, binding, transport) = up to 5 marks
• Correct sequence and leg specificity = 4 marks
  Teaching note: Must show continuous route and passive diffusion at exchange sites; no active transport for O₂ entry.

End of Answer Key